1. Field of the Invention
This invention can be adapted for use in almost any kind of data storage system where data sequences are recorded in specified areas of the recording medium by relative motion between it and a magnetic transducer head. In the present state of technology the most common embodiment of this invention will probably be in a magnetic disc recording system where the recording medium is a rotating disc, and the magnetic head is mounted on an arm which moves the head radially to access various data tracks on the disc as desired. A variation has applications in strip recorders for positioning the strip relative to the head.
In high density disc recording apparatus for example, concentric recording tracks have radial densities from 200 to as many as 1,000 tracks per inch. It has been found to be almost impossible to record and read data having such track densities without having a fixed set of references on the disc, or on at least one of the discs in a multi-disc system, which can be used for positioning the head or arms. In one design having several discs carried on one spindle, one disc surface is devoted to carrying these reference marks, which in fact comprise a series of concentric servo tracks read in a fashion similar to data tracks, but used only for positioning the head arm assembly.
Several problems have arisen when such as arrangement is used in high track density systems. First of all, it is obvious that one recording surface is lost for data recording purposes because it is dedicated to servo tracks. Secondly, and more importantly, errors in alignment of the data head arms with respect to the servo head arm plus errors in the spindle and disc location and dimensions impose an upper limit on radial track density, which if exceeded results in data readback errors.
2. Description of the Prior Art
Several different patents disclose recording media formed from two distinct layers of differing coercivities. U.S. Pat. No. 3,219,353 (Prentky) discloses a recording medium usable in the system which we disclose, and discusses various considerations involved in its use. Prentky comprises the best art of which applicants are aware. U.S. Pat. No. 2,691,072 (Mathes) discloses a magnetic recording medium having two magnetic layers of differing coercivities. In addition, a large number of patents disclose systems employing a recording medium having at least one servo track which may be read and the signal used to position a read or write head adjacent the recording medium. See, e.g. U.S. Pat. No. 3,593,331 (Connell, et al.) and U.S. Pat. No. 3,656,130 (Bucklin, et al.)